1. Field of the Invention
This invention relates to optical imaging systems and specifically concerns a holographic illumination system to create a continuous self-luminous source of arbitrary shape and numerical aperture.
2. Description of the Prior Art
The drive toward higher density circuitry in microelectronic devices has promoted interest in a variety of high resolution lithographic techniques which require the ability to produce finer resolution patterns at high production rates. In optical lithography, the improvement in resolution that results by use of shorter wavelengths is well known. As a result of these considerations, an effort has been made to develop processes and materials which require exposure in the deep UV spectral region. The light source traditionally used in these prior art systems has been either a deuterium lamp or a xenon-mercury arc lamp. The problem with using such lamps is that insufficient power is available from them in the desired spectral region. For a typical lamp in a typical system, the total deep UV power than can be collected for use is in the few tens of milliwatts range, so that the exposure time for photoresists that are sensitive in the deep UV are typically several minutes. A copending application Ser. No. 268,511, filed May 29, 1981 by Jain et al, (now abandoned) discloses and claims optical lithography apparatus having a pulsed excimer laser light source. This apparatus is capable of producing uniform exposure of a pattern at very fine resolution in an exposure time substantially faster than prior art systems. The pulsed excimer laser light source disclosed in that application produces a nearly collimated beam which is unsuitable for use in prior art projection systems which have been designed to utilize illumination through an arbitrary shape such as a curved slit.